An important class of carbon nanotubes is single-walled carbon nanotubes (SWCNTs). They are generally produced as ensemble samples containing both metallic and semiconducting nanotubes with a distribution of chiralities centered at a mean diameter. Several methods can be used to produce SWCNTs which will vary in the distribution of chiralities, diameter range, semiconducting/metallic (sc/m) content and average length. For example, HiPco and CoMoCat SWCNTs are relatively smaller in diameter (0.6-1.3 nm), while arc-discharge, laser (laser-ablation) and plasma SWCNTs are relatively larger (1.0-2.2 nm). Though a sc-SWCNT content as high as 95% can be produced with techniques such as CoMoCat, most as-prepared SWCNT samples have less than 70% sc-content. For many applications, such as thin film transistors (TFTs), a sc-purity higher than 99% is needed, therefore scalable methods that enable a commercially viable process need to be developed.
Several methods have been used to demonstrate the effective enrichment and isolation of semiconducting SWCNTs with greater than 99% sc-purity as assessed by absorption spectroscopy. Among these methods are density gradient ultracentrifugation (DGU), gel chromatography (GC), dielectrophoresis and selective extraction by conjugated polymers. Amongst these listed options, chromatography and conjugated polymer extraction may provide a clearer path to scalable enrichment of sc-SWCNTs. Furthermore, the simplicity of the conjugated polymer extraction process, which generally entails a dispersion followed by a centrifugation step, further distinguishes it from the rest as a cost-effective method for the isolation of sc-SWCNTs with greater than 99% semiconducting content.
The first disclosure that conjugated polymers could selectively disperse semiconducting SWCNTs and lead to enriched semiconducting SWCNT fractions of relevance for thin film transistor fabrication can be found in the patent literature (Malenfant 2007). Subsequently, the exceptional selectivity that could be achieved with polyfluorene derivatives towards specific semiconducting SWCNT chiralities was demonstrated. More recently an effective enrichment of HiPco sc-SWCNTs using poly(3-dodecyl thiophene) (P3DDT) and arc-plasma-jet tubes using PFDD was also demonstrated to provide TFTs with mobilities greater than 10 cm2/Vs. Collectively, these results, amongst others, have clearly shown the potential for conjugated polymers in sc-SWCNT enrichment and TFT device fabrication.
To date, many homo- and copolymers of phenylenevinylene, carbazole, thiophene and fluorene have been investigated for enrichment. For example, P3DDT displayed a promising result in the separation of HiPCO nanotubes, however P3DDT is not suitable for the separation of larger diameter SWCNTs, which are more desirable when trying to minimize contact resistance and to obtain a large electron mobility in thin film transistors. Similarly, it has been observed that poly(9,9-dioctylfluorene) (PFO) has a high selectivity in dispersing small-diameter sc-SWCNTs with large chiral angles (20°≤θ≤30°), but not large-diameter SWCNTs, which is believed to be difficult to disperse and to enrich owing to the strong interaction between the nanotubes associated with the low curvature of the nanotube wall. As a result, co-monomer units have been introduced into the polyfluorene main chain in order to target specific tube chiralities/diameters. They include: phenylene-1,4-diyl, thiophen-2,5-diyl, anthracene-9,10-diyl, anthracene-1,5-diyl, naphthalene-1,5-diyl, 2,2-bithiophene-5,5′-diyl, and benzo-2,1,3-thiadiazole-4,7-diyl.
Furthermore, the length of the side alkyl chain of the polymers has a significant impact on the enrichment effectiveness. Polymers with 12-carbon long side chains showed an improved selectivity to sc-SWCNTs with larger diameters. Recently work on the enrichment of large diameter SWCNTs using fluorene homopolymers with long alkyl side chains has been done, which achieved a device performance of 14.3 cm2/Vs and on/off ratio over 105.
Current enrichment methods are limited by a combination of issues such as a lack of scalability (DGU), prohibitive cost (chromatography), the yield/effectiveness and device performance (selective polymer extraction).
There remains a need for commercially viable processes for separating sc-SWCNTs in high yield and high purity from m-SWCNTs.